Electric oscillator



Cil

r ing element.

Patented Jan. 3, 1939 UNITED STATES PATENT OFFICE ELECTRIC OSCILLATORApplication September 2, 1936, Serial No. 99,051

6 Claims.

This invention relates to electric oscillators. Its principal object isto improve the frequency stability of oscillators operating atWave-lengths of a meter or less.

It has been possible to construct a relatively stable electricoscillator by using a Lecher circuit or concentric transmission lineconnected to the grid-cathode circuit of an electron discharge device asthe tuning or frequency stabiliz- It has been found that the frequencystability is increased with increased length of line so that athree-quarter wave-length line renders the circuit more stable than aquarter Wave-length line.- A five-fourths Wave-length e line gives stillgreater stability than a three-quarter Wave-length line.

There is, however, considerable difficulty and expense involved inbuilding long transmission lines. Moreover, these lines make theapparatus unduly large. It is therefore desirable if a transmission linebe used as a stabilized tuning circuit of an oscillator, to make it asshort as possible. However, with the shorter transmission line sectionthe irequency is not suiiiciently stable.

Another problem which is involved is that of suitably coupling thetransmission line to the electron discharge tube. With the shortestpossible connections there is still a length of lead Wire between theelectrode of the discharge device and the transmission line conductor towhich it is connected. Moreover, the electron discharge device WillalloW current to iioW from the grid to the cathode While oscillatingthereby producing power loss. In addition to the lead Wires and gridleak circuits which introduce loss and effective resistance in theassociated circuits, there is a damping effect introduced by what isknown as active grid loss. If, therefore, the transmission line isclosely coupled to the electron discharge tube vthese various losses arereected into the transmission line by the coupling so that theyeffectively introduce resistance into the transmission line itself thusgreatly decreasing its selectivity. An important feature of theinvention is the provision of means to so couple the transmission lineto the electron discharge device as not to introduce into thetransmission line substantial amounts of resistance.

The extent to which an electrical element such as an inductance coil ora capacity element tends to discriminate between currents of differentfrequencies is dependent primarily upon the magnitude of its reactancewith respect to that of its resistance. If the selectivity factor of anin- (Cl. Z50-36) ductance coil, that is, the ratio of its reactance toits resistance be denoted by Q we may write Where o is the angularvelocity at the frequency under consideration, L is the inductance and Rthe series resistance. Similarly, for a condenser 1 Q wCR (2) 10 in thec-ase of a tuned circuit which comprises equal component reactance ofopposite sign or a net reactance which is substantially zero, We maytake as a measure of the selectivity or Q l5 of the circuit the ratio ofthe either reactance component to the effective series resistance of thecircuit. In other Words, either Equation 1 or Equation 2 may be appliedto the case of a tuned circuit. In this specification and the appendedclaims, the expression selectivity factor and Q will be usedinterchangeably with this signicance.

In accordance with the invention, an electric discharge device Which maybe of the Well-known type having a cathode and an impedance control gridis provided With an input circuit comprising reactance elements which,together with the inherent grid-cathode capacity, tune the input circuitto the desired operating frequency in a certain desired manner. Afrequency stabilizing circuit comprising a quarter wave-length high Q orlow-loss line short-circuited at one end is associated with the vacuumtube oscillator by a loose coupling to the tuned input circuit. Thecoupling involves a step-down impedance transforming connection of suchcharacter that the resistances inherent in the electrode leads and gridleak and polarizing circuits and the virtual resistance represented bythe active grid loss are greatly reduced insofar as they eifectivelyenter into the selectivity of the quarter Wave-length line. It resultsfrom this that the selective characteristic of the line remains verysteep in its resonance region and the line is so little affected by theimpedances of the tube circuit to which it is coupled, that its inherentinductance constitutes a very large part of the total inductancedetermining its natural resonance frequency. The oscillator consequentlyis stabilized at an operating frequency very close to the naturalresonance of the line and therefore in a region in which its reactancecharacteristic is extremely steep, so that any tendency to change theoscillator frequency is opposed by a very large change in the reactanceof the line. It thus becomes possible to utilize the inherent high Q ofa quarter Wave-length line without impairment by the resistances of theassociated tube circuits.

Other features and aspects of the invention will be apparent from aconsideration of the detailed description and appended claims taken inconnection with the annexed drawing in which:

Fig. 1 illustrates the approximate equivalence of a quarter wave-lengthline and a tuned circuit;

Fig. 2 portrays the reactance frequency characteristic of a quarterwave-length line shortcircuited at one end;

Fig. 3 illustrates a transmission line directly connected to the inputelectrodes of the vacuum tube and co-acting therewith to constitute aquarter wave length line;

Fig. 4 is a diagram to assist in explaining the operation of a couplingcircuit;

Fig. 5 shows schematically a circuit in which the quarter wave-lengthline of Fig. 3 is coupled to the vacuum tube by a tuned circuit; and

Fig. 6 illustrates diagrammatically the schematic of the preferred formof oscillator circuit including the impedance transforming connectionaccording to the invention.

A quarter'wave-length line with one end shortcircuited as indicated inFig. 1 may, for certain purposes, be viewed from its open end as thesubstantial equivalent of a tuned circuit having the inductance andcapacity indicated by dotted line elements. In the region of theresonant point of the tuned circuit its performance characteristic issimilar in general form to that of a quarter wave-length line. Atfrequencies remote from the resonance frequency the performances of thetwo circuits are different, but Within the region in which thetransmission line is to be used as a frequency stabilizer for anoscillator in accordance with the principles of this invention, theparallel resonant circuit and the short-circuited quarter wave-lengthtransmission line may theoretically be regarded as equivalents. This isshown in Fig. 2 which is a graph of the reactance curve of either theshort-circuited quarter wavelength transmission line or the parallelresonant tuned circuit from zero frequency up to a frequency beyond theresonance point.

Fig. 3 represents the equivalent circuit of a transmission lineshort-circuited at its remote end and connected to the electrodes of anelectric discharge device to operate as a quarter wave-length line.Inasmuch as the cap-acity between the grid and cathode is equivalent toa considerable portion of the entire capacity involved, the resultantresonance frequency will not be that of Fo the natural frequency of thequarter wave-length line, but will fall at F1, a considerable distancefrom the resonance frequency of the transmission line per se and atwhich it has a reactance measured by the ordinate of the reactance curveat frequency F1 up to the point P1 on the curve. It will be apparentthat the rate of change of reactance at frequency F1 with change infrequency which is measured by the slope of the curve at point P1 is notlarge and this in spite of the fact that the selectivity factor or Q ofthe connected transmission line per se may be very high.

The ideal vacuum tube would among other things have a grid of extremelysmall capacity and grid-cathode leads of zero reactance and resistance.Under such conditions, attaching the transmission line as in Fig. 3would enable operating at point Pz on the steep part of the graph ofFig. 2. The capacity of the grid and the inductance of the lead wires,however, have such low reactance to resistance ratios compared to thecorresponding ratio of the transmission line that if included in thefrequency determining stabilizing circuit, they will seriously reduceits selectivity.

In accordance with the present invention, there is provided a couplingcircuit which will largely eliminate the deleterious effects of theseinter-electrodes and lead circuit reactances. If a suitable transmissionline is connected to the grid and the cathode of the vacuum tube as inFig. 4 such that with the grid capacity and the lead inductances addedit constitutes the equivalent of a half wave-length line, the grid andlead reactances will merge With those of the transmission line and acircuit connected to the remote terminals I3, I4 of the transmissionline will be presented with the same impedance as if connected directlyto the grid and cathode of the tube, but with the grid-cathode capacityand the lead reactance eliminated at the operating frequency. Thestabilizing circuit may accordingly be connected to terminals I3 and I4and will be in effect directly connected to a grid and cathode.

In case it is desired to use a more compact form of oscillator, thetransmission line type of coupling may be replaced by a circuitinvolving lumpedA reactances as illustrated in Fig. 5. As shown in Fig.5 an oscillator system involves a tuned circuit including the elements Iand 2 connected up to serve as an equivalent of the half wave-lengthcoupling transmission line and at the same time to avoid excessivephysical magnitude. The element I has a capacity approximatelyequivalent to the grid-cathode capacity which is represented in dottedline at 3.

A lumped inductance 2 is given such magnitude to the operatingfrequency, and including the primary winding of a transformer 8 by whichpower is supplied to the load circuit 9. In this oscillator system, thefrequency stabilizing transmission line 4 is so connected to the inputelectrodes of the vacuum tube that operation can occur at point P2 or atany other desired part of the curve in Fig. 2 and at a frequency F2which is near to the natural frequency of the stabilizing line. Thisarrangement enables the stabilizing transmission line to develop ahighly selective characteristic and to utilize its most effectiveoperating range relatively unimpaired by gridcathode reactances.

Power losses in the grid-cathode circuit still remain and appear as aresistance between terminals I3 and I4. The effective resistance whichso appears is usually such as to impair the high selectivity or Q of thelow loss transmission line when connected as in Fig. 5. It is thereforeanother object of this invention to so connect the stabilizingtransmission line to the terminals I3, I4 that this resistance makeslittle change in the Q of the stabilizing lines and produces nonoticeable effect upon its reactance.

It is, of course, desirable to use a stabilizing circuit having a Q orselectivity factor as high as it is practicable to obtain. For thisreason a short-circuited quarter wave-length coaxial line section asindicated at l in Fig. 6 may serve to great advantage. With such a highselectivity circuit it is undesirable to reduce its selectivity bycouplings which introduce dissipative impedances between the terminalsof the high selectivity circuit at its open end. According to a featureof this invention, coupling to the coaxial line section is effected byconnecting one terminal of capacity element l to the outer conductor ofsection I D and the other terminal to the inner conductor at a point ashort distance from the short-circuited end, the connection to the innerconductor passing through an opening Il in the outer conductor. 'I'hisconnection reduces the portion of the total reactance of the coaxialline which is presented across the terminals of capacity element I. Thereduction is, however, more than compensated for by the twocounteracting effects of increase in effective height and steepness ofthe reactance-frequency characteristic of the coaxial line in itsresonance region and the further approach of the operating frequency F2toward the resonance frequency Fo. 'I'he circuit of Fig. 6 is thereforeextremely stable from a frequency standpoint and involves apparatuswhich is neither bulky nor expensive.

In order to bias the grid to the proper operating potential a C batteryl2 may be provided, but it will be understood that the grid leak device5 of Fig. 5 may be substituted if desired.

Although the features of the invention have been illustrated anddescribed in particular circuit embodiments, it is to be understood thatthe invention is not to be limited thereto, but only by the scope of theappended claims.

What is claimed is:

1. In combination, an electron discharge device having a cathode, ananode and an impedance control element, a tuned output circuitconnecting the anode and cathode, a tuned input circuit connecting thecathode and the impedance control element and a high selectivity circuitcomprising a quarter wave-length line shortcircuited at one end andhaving connections from its elements at a point near the short-circuitedend to the tuned input circuit.

2. In combination, a quarter wave-length section of coaxial lineshort-circuited at one end, an electric discharge device having acathode, an anode and an impedance control member, an output circuitconnecting the anode and cathode, a tuned input circuit connecting thecathode and impedance control member, connections from two points on theinput circuit to the inner and outer conductors, respectively, of thequarter wavelength section, the connection points of the quarterwave-length section lying intermediate its ends and closer to theshort-circuited end than to the other end.

3. An oscillator comprising an electric dis- Y charge amplifier havinginput electrodes and output electrodes, a space current path connectingthe output electrodes, a circuit tuned effectively to the desiredoscillation frequency connecting the input electrodes, a coaxial line ofquarter wave-length at the desired operating frequency having one endshort-circuited, a lead from the inner conductor of the coaxial lineconnected thereto at a point relatively close to the shortcircuited endand passing out through an aperture in the surrounding conductor, asimilarly located lead connected to the outer conductor, and meansconnecting the leads to the tuned circuit at electrically separatedpoints to reduce the impedance introduced into the tuned circuit withrespect to that presented by the leads at their terminals.

4. In combination, an electron discharge device having a cathode, ananode and an irnpedance control element, a tuned circuit connecting theimpedance control element and the cathode, a frequency stabilizingcircuit having a natural frequency substantially equal to the resonancefrequency of the tuned circuit and having an extremely high ratio ofreactance to effective resistance, electrical connections leadingdirectly from two points in the frequency stabilizing circuit to twopoints in the tuned circuit, the reactance between the two points in thestabilizing circuit being relatively low compared with the entirereactance of the stabilizing circuit and the reactance between the twopoints in the tuned circuit being relatively low compared with the totalreactance of the same kind in the tuned circuit whereby the tunedcircuit and the stabilizing circuit are loosely coupled in order todiminish the impedance which is effectively introduced from the tunedcircuit into the stabilizing circuit so as to achieve high selectivityin the stabilizing circuit, and an output circut connected between theanode and cathode and uncoupled to the frequency stablizing circuitexcept through the inherent coupling Within the electron dischargedevice.

5. An oscillator comprising an electric discharge device having acathode, an impedance control element and an anode, an input ,circuitpath connected between the cathode and impedance control element, anoutput circuit connected between the cathode and anode, a frequencystabilizing circuit consisting of a coaxial line of effectivelyone-quarter wave-length at the operating frequency of the oscillationsto be produced and short-circuited at one end, a capacity elementconnected in series in the input circuit path, and leads connecting itsterminals, respectively, to the two conductors of the coaxial line sothat the frequency stabilizing circuit is so loosely coupled to theinput circuit that its inherent high ratio of reactance to resistance isnot seriously impaired by impedance effectively introduced by thecoupling to the input circuit.

6. An oscillator comprising an electric discharge device having acathode, anode and impedance control element, an output circuitconnecting the cathode and anode, a path connecting the cathode andimpedance control element including in series a capacity element havinga capacity approximately equal to that between the cathode and theimpedance control element, and an inductance having a magnitude suchthat with these capacities it tunes the path to the operating frequency,and a frequency stabilizing transmission line of high selectivityconnected to the terminals of the series capacity element.

RAYMOND A. HEISING.

